The invention relates to an optical system for a projector and an imaging method thereof, and in particular to an optical system occupying relatively smaller space by enabling light to pass through a lens set twice.
A light beam in a projector passes through a color wheel to emit a desired primary color light, such as red, blue or green light. The primary color light passes through a series of optical lenses and is projected onto a digital micromirror device which determines pixel positions according to a control system. The primary color light beam from the digital micromirror device is projected onto a screen by a projection lens set. The color wheel creates light beams of three primary colors, and the digital micromirror device determines pixel positions of the primary color light beams respectively to form an image.
Projectors are substantially divided into two types according to the position of the optical axis of the digital micromirror device and the axis of projection lens set:
In the first type, the optical axis of the digital micromirror device is aligned with the axis of the projection lens set as shown in FIG. 1. In the second type, the optical axis of the digital micromirror device has a predetermined offset angle with respect to the axis of the projection lens set as shown in FIG. 2.
In FIG. 1, a light beam from a luminous body 12 is focused by a condenser 14, passes through a color wheel 16 to create a desired primary color light. The light is made uniformed by a light integrator 18, and passes through a first lens 22, a second lens 24 and a prism 90 comprising two triangular prisms. Prism 90 reflects the light beam to a digital micromirror device 60 which reflects the light beam to the projection lens set 70. In the described structure, as the prism 90 reflects the light beam to the digital micromirror device 60 and allows the light beam reflected by the digital micromirror device 60 to pass therethrough, the digital micromirror device 60, projection lens set 70 and prism 90 are substantially aligned to save space. The light beam reflected by the digital micromirror device 60 has no deviation from the projection lens set 70 so that the light beam from the digital micromirror device 60 can be projected onto a screen via the projection lens set 70. The prism 90, however, is costly and has considerable weight, and is difficult to ship.
In FIG. 2, a light beam passes through a first lens 22, a second lens 24 and a third lens 30. A mirror 80 reflects the light beam to the digital micromirror device 60. As the light beam cannot pass through the mirror 80, the digital micromirror device 60 must have an offset angle with respect to the projection lens set 70 to prevent interference. Although no prism is used in the structure, the configuration of the digital micromirror device 60, the projection lens set 70 and the mirror 80 may possibly occupy more volume.